The broadest objectives of this proposal are to reveal the molecular basis for the voltage sensitive ionic permeability properties of excitable membranes. the approach assumes that membrane ion permeation sites called channels are intrinsic proteins and that procedures which modify the higher order structure of proteins will alter permeability properties. The particular method utilized here is dye-sensitized photochemical modification of lobster giant axon membranes, with a voltage-clamp analysis of ionic permeability properties using the double sucrose-gap technique. The procedure will modify axon membranes under conditions similar to those employed by other investigators for the photochemical modification of proteins in solution. A correlation of altered permeability properties with the most likely alteration of membrane molecules will provide new information relevant to the molecular basis for excitation.